General formatting improvements in the spec

The spec always needs some love, so here I go:

**This is only about formatting, it does not change content or even wording.**

 * syntax highlighting for scala code, using highlight.js: 
    * this is done in a way that *interacts nicely with MathJax*: one can use latex inside code blocks without problem (and still get syntax highlighting),
    * uses the latest highlight.js 8.2, with the Scala support freshly revamped by @non,
 * individual HTML pages now have titles,
 * no more broken links,
 * fix or work around some latex problems with MathJax,
 * mass-removal of trailing whitespace, while I was at it...

I uploaded a preview of the result [here](http://static.antoine.gourlay.fr/temp/spec/) to make reviewing easier.